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Digital design interview questions

617 questions across every lesson, from basics to the tricky ones. Think first, then reveal the answer, the code and the animation that shows it.

What is the range of a 6-bit signed number?

Why does two's complement dominate?

How many different patterns can a 10-bit bus carry, and what range does it cover when read as unsigned?

Write -1 as an 8-bit and as a 16-bit two's complement value, in hex.

The byte 0xB6 appears on a bus. What is it in binary, as an unsigned number and as an 8-bit signed number?

Apply the invert-and-add-one rule to -128 in 8 bits. What do you get, and why?

A designer widens a signed 8-bit sample to 16 bits as shown. The sample holds -13. What does `w` hold, and how do you fix it?

What is the smallest two's complement width that can hold every integer from -100 to +100?

In this Verilog, `sa` holds -3 and `ub` holds 5. Is `sa < ub` true or false, and why?

You must narrow a 16-bit signed value `x` to 8 bits and raise a flag when it does not fit. How do you test whether it fits?

How many 2-input NANDs make an XOR?

Why are NAND gates preferred over NOR in CMOS?

How many different Boolean functions of two inputs exist?

A 3-input XOR has all three inputs at 1. What is the output?

Build a 2-input OR using only 2-input NAND gates. How many do you need?

Show how NOR alone makes an inverter and a 2-input AND.

A 5-input XOR's truth table has how many rows, and in how many of them is the output 1?

What does this line compute for two 4-bit buses, and why does it work?

An 8-bit byte is 1101_0110. What even-parity bit is sent with it, and what does the receiver check?

Can you make a 3-input NAND by feeding one 2-input NAND's output into a second 2-input NAND with the third input?

Simplify A + A'B.

State De Morgan for (AB)'.

Simplify A(A + B).

Apply De Morgan to (A + B')'.

Show that AB + A'C + BC equals AB + A'C. Why might a designer keep the BC term anyway?

Rewrite F = AB + CD so that it uses only NAND gates.

The comment says what the designer wanted. What does the code actually build, and how do you fix it?

In ordinary arithmetic, a + bc does not equal (a + b)(a + c). Is A + BC = (A + B)(A + C) true in Boolean algebra?

What is the dual of the identity A + A'B = A + B, and is the dual also true?

Simplify F = (A + B)(A' + C) to a sum of products.

Why Gray order on the axes?

A 4-variable K-map has how many cells, and how many literals does a group of four cells give?

Why can't a K-map group contain three cells?

Minimise F(A, B, C) = sum of minterms 0, 2, 4 and 6.

On a 4-variable map with A, B down the side and C, D across the top, only the four corner cells are 1. What is the minimal expression?

F(A, B, C) is 1 for minterms 1, 3 and 7, and minterm 5 is a don't-care. Minimise it with and without the don't-care.

You fill a 4-variable map and get a perfect checkerboard of 1s. What does that tell you?

A colleague labels the BC columns of a 3-variable map 00, 01, 10, 11. Row A = 0 has 1s in the two middle columns, and they circle them as one pair. What is wrong, and what is the correct expression?

A flag for 'BCD digit is 5 or more' was minimised with codes 10 to 15 as don't-cares, giving F = A + BD + BC. A reset fault leaves 1100 on the bus. What does the flag show, and what is the lesson?

Why is static CMOS low in static power?